The field of this invention relates to wireless communication units, integrated circuits, transmitter architectures and in particular circuits for providing a differential to single ended conversion of signals. The invention is applicable to, but not limited to, envelope tracking using a differential circuit configuration and a method therefor.
A primary application of the present invention is in the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of these linear modulation schemes fluctuate, this phenomenon results in the average power delivered to the antenna being significantly lower than the maximum power, potentially leading to poor efficiency of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier. Linear modulation schemes require linear amplification of the modulated signal in order to minimize undesired out-of-band emissions from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device typically be approximated by a straight line, i.e. as in an ideal linear amplifier case. This ‘linear’ mode of operation provides a low efficiency of DC to RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, such low efficiency performance is also recognised as being problematic for the base stations.
Furthermore, the emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearization techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearizing techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion.
In order to increase the bit rate used in transmit uplink communication channels i.e. communication channels from the subscriber communication unit to a serving base station, larger constellation modulation schemes, with an amplitude modulation (AM) component are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-point quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity.
To help overcome such efficiency and linearity issues a number of solutions have been proposed. One technique used to overcome efficiency and linearity problems relates to modulating the PA supply voltage in order to match the envelope of the radio frequency waveform that is being transmitted by the RF PA. Proposed solutions that utilise envelope modulation include envelope elimination and restoration (EER), and envelope tracking (ET).
It is known that the use of PA supply RF envelope tracking may improve both PA efficiency and linearity for high peak-to-average power ratio (PAPR) high-power transmit conditions. FIG. 1 illustrates a graphical representation 100 of two alternative techniques; a first technique that provides a fixed voltage supply 105 to a PA, and a second technique whereby the PA supply voltage is modulated to track the RF envelope waveform 115. In the fixed supply case, excess PA supply voltage headroom 110 is used (and thereby potentially wasted), irrespective of the nature of the modulated RF waveform being amplified. However, for example in the PA supply voltage tracking of the RF modulated envelope case 115, excess PA supply voltage headroom can be reduced 120 by modulating the RF PA supply, thereby enabling the PA supply to accurately track the instant RF envelope.
Envelope tracking may also support a high-efficiency improvement potential for high PAPR conditions, which in turn may lead to less DC power being dissipated. As a consequence, heat is reduced and the PA may operate at a cooler temperature for the same output power. However, it is also known that for high bandwidth signals, accurate tracking of the RF envelope is difficult to achieve in practical implementations. Dependent upon the overall system architecture, the bandwidth of the supply modulator may be significantly (for example two to five times) greater than the signal (envelope) bandwidth in order to minimize the impact of the modulator group delay through time alignment, etc.
It is known that differential circuit implementations may provide advantages with respect to noise, immunity and headroom over single ended circuit implementations. FIG. 2 illustrates graphically 200 a simplified overview of a differential envelope/modulated tracking signal, whereby a differential envelope/modulated waveform 210, comprising a positive input (Vinp) 215 and a negative input (Vinn) 220, is superimposed on a fixed DC signal 225. As illustrated, the differential envelope/modulated tracking signal comprises a DC portion and an AC portion, whereby both portions are passed through any subsequent differential circuit element.
FIG. 3 illustrates a known simplified modulator circuit 300 used for envelope tracking. The simplified modulator circuit 300 comprises a differential to single ended conversion arrangement 305 having a differential input 310 with a negative envelope signal input (N) 315 and a positive envelope signal input (P) 320. The differential to single ended conversion arrangement 305 converts the differential input 310 to a single ended output that is input to a first linear, Class AB, amplifier stage 325. The amplified signal is then combined with the output of a second, e.g. Class D, power amplifier stage 335, which uses a current sense 330 to sense the zero crossing of the first stage 325 and, together with hysteresis, control the current from the class D stage 335. In this manner, both AC and DC signals are passed through the signal path.
However, the simplified modulator circuit 300 is not ideal in that the input signal corresponding to the envelope is strictly positive. Thus, the potential headroom benefits due to employing a differential input signal (i.e. both positive and negative portions of the signal) are not realized, even when employing additional circuit techniques to minimize DC offset and/or reduce noise. A further disadvantage in the simplified modulator circuit 300 is that the input AC envelope signal is not symmetrical around a mean value. Often, complex techniques such as de-troughing (or other signal mapping techniques) may then be required to increase the asymmetry of the modulator signal, thereby increasing the complexity of circuit 300.
Thus, a need exists for an improved circuit, for example an integrated circuit comprising a modulator for use with a differential interface for an envelope tracking signal, a wireless communication unit and a method therefor.